The circumstances are unclear and disputed. There were initial
reports that he may have mistaken Steenkamp for a burglar.
And according to one report, this may have been the result of
a Valentine's Day surprise.

Just yesterday, Steenkamp tweeted about having something up her
sleeve:

However, other reports, and the police dispute the idea of a
"surprise." He is
being held without bail and there may have been previous
domestic incidents.

Still, the story remains a mystery. In an interview with the NYT
last year,
Pistorius expressed his enthusiasm for guns, and even talked
about tiptoeing downstairs at his home, thinking his house was
being invaded by an intruder.